1. Field of the Invention
The present invention relates to a fluidized-bed classifier for classifying a particulate material pre-ground by a roller mill for grinding cement clinker or the like by particle size.
2. Description of the Related Art
FIG. 5 shows the constitution of a cement clinker pre-grinding system disclosed in JP-A No. 7-108187. A fluidized-bed classifier 1 separates coarse particles from the material pre-ground by a roller mill through the agency of a fluidized bed, and returns the separated coarse particles to the roller mill 2 for regrinding. The fine particles separated from the particulate material by the fluidized-bed classifier 1 is fed to a tube mill 4 for further pulverization. The ground material pulverized by the tube mill 4 is conveyed upward by a bucket elevator 5 into a separator 6. The separator 6 separates fine particles from the ground material and delivers the fine particles as a product. Since the material fed to the tube mill 4 is obtained by separating coarse particles from the pre-ground material produced by the roller mill 2, the tube mill 4 may be provided with balls of a relatively small diameter and thereby the energy demand of the cement clinker pre-grinding system can be reduced. The roller mill 2 is replenished with cement clinker fed from the material hopper 7, and the cement clinker fed from the material hopper 7 is ground together with the coarse particles returned to the roller mill 2 from the fluidized-bed classifier 1.
FIG. 6 is a perspective view of the fluidized-bed classifier 1 included in the cement clinker pre-grinding system of FIG. 5. The internal space of a vessel 10 is partitioned into an upper fluidized-bed chamber 12 and a lower air chamber 13 by a perforated dispersion plate 11. Air is supplied through an air inlet 14 into the air chamber 13, and the air is discharged from the fluidized-bed chamber 12 through an air discharge duct 15. The vessel 10 is provided with a material feed chute 16 opening into the fluidized-bed chamber 12. A pre-ground material, such as cement clinker, is supplied through the material feed chute 16 into the fluidized-bed chamber 12, and then the pre-ground material is fluidized in the fluidized-bed chamber 12 by currents of air supplied into the air chamber 13 and blown through the perforations of the dispersion plate 11. The dispersion plate 11 is inclined at an inclination a so as to slope down from a side wall of the vessel 10 on the side of the material feed chute 16 toward a discharge side wall of the same. An upper chute 17 and a lower chute 18 are joined to an upper portion and a lower portion, respectively, of the discharge side wall of the vessel 10. The pre-ground material fed through the material feed chute inlet 16 is fluidized by current of air blown through the perforations of the dispersion plate 11 and flows along the inclined dispersion plate 11 toward the discharge side wall. Fine particles of the material are fluidized and flow upward in the fluidized bed into the upper chute 17, while coarse particles of the material are not fluidized at all, or even though they are fluidized they do not come up to the surface and slide along the inclined dispersion plate 11 into the lower chute 18. Accordingly, it is expected that fine particles are discharged through the upper chute 17 and coarse particles are discharged through the lower chute 18.
This known fluidized-bed classifier 1 shown in FIG. 6 is unable to separate fine particles and coarse particles satisfactorily from each other, because both the upper chute 17 and the lower chute 18 through which fine particles and coarse particles are discharged, respectively, from the vessel 10 are joined to the upper and the lower portion, respectively, of the discharge side wall of the vessel 10. Consequently, fine particles containing some coarse particles are discharged through the upper chute 17 into the tube mill 4 (FIG. 5), and hence the tube mill 4 needs balls of a diameter large enough to crush large particles mixed in small particles. The material has a very wide range of particle size distribution of ten-odd millimeters to several micrometers. Therefore, if such a material is to be fluidized to separate fine particles of particle sizes below several millimeters from the material, coarse particles of particle sizes greater than an upper limit particle size need to be fluidized and hence coarse particles having particle sizes greater than the upper limit particle size are included in fine particles discharged through the upper chute 17.
Since the dispersion plate 11 is a single, inclined, perforated plate, the difference in height between one end of the dispersion plate 11 on the side of material feed chute 16 and the other end of the same on the side of the upper chute 17 and the lower chute 18 is large if the fluidized-bed classifier 1 has a large size. Since the height of the upper stratification of the fluidized bed formed over the dispersion plate 11 is fixed, the difference in thickness between a portion of the fluidized bed on the side of the material feed chute 16 and a portion on the side of the upper chute 17 and the lower chute 18 is very large. Accordingly, it has been difficult to construct the fluidized-bed classifier 1 in a large size.